Polyolefin grafted copolymers represent an interesting class of copolymers because they may have some properties typical of the grafted polymer and at the same time some properties typical of the polyolefin backbone. It is known that certain physico-mechanical properties of polyolefins can be improved by blending with e.g. amorphous polymers such as polystyrene. However, physical blends of such polymers are generally incompatible, owing to the high surface tension and poor adhesion between the immiscible polymers in the blend. For this reason, physical blends require the use of a compatibilizer to reduce the above-mentioned problems.
Better results with respect to the physical blends are obtained when the modifying (co)polymer is "chemically" blended with the polyolefin, i.e. when the modifying (co)polymer is grafted onto the polyolefin backbone. When compared to physical blends of polymers, graft copolymers usually exhibit a finer heterophasic morphology in which the domain size of the dispersed phase is resistant to coalescence in subsequent processing and may be smaller by about one order of magnitude. In addition, the necessary adhesion between the polyolefin backbone polymer and the modifying grafted (co)polymer derives from the chemical covalent bond between the backbone polymer and the graft (co)polymer rather than on the action of an external compatibilizing agent. Depending on the intended results, different types of polymerizable monomers can be used for the preparation of the grafted (co)polymer, including e.g. styrene and in general aromatic vinyl compounds, acrylic compounds, acrylonitrile, etc.
Polyolefin grafted copolymers can be used as stand-alone structural plastic or can be blended with other grafted or ungrafted polymers to further improve or provide additional properties. Examples of olefin polymer graft copolymers and blends prepared therefrom are described e.g. in U.S. Pat. Nos. 4,990,558, 5,370,813, 5,473,015, 5,310,794, 5,286,791, 5,447,985.
Graft copolymers can be prepared by creating active sites on the main olefin polymer chain or backbone, and initiating graft polymerization of a polymerizable monomer at these sites. Procedures which have been used for introducing such active sites into the backbone have included treatment with organic chemical compounds, such as peroxide or azo compounds, capable of generating free radicals, and irradiation.
Of the various processes which have been employed for preparing polyolefin grafted copolymers, the so called "dry" process, such as is carried out in a mechanically stirred reactor, and this gas phase process are more efficient than the processes which use a liquid suspending medium or solvent because of its high conversion, reduced by-product formation, reduced environmental impact and lower manufacturing costs. The gas phase process of this invention also provides process simplicity, reduced fouling, improved mixing between the ingredients, and high heat transfer surface per unit reaction volume.
A method of producing olefin polymer graft copolymers, which overcomes the above problems, is described in U.S. Pat. No. 5,140,074. In said method, the grafting reaction is controlled, inter alia, by maintaining the rate of addition of the grafting monomer below 4.5 pph (parts by weight per 100 parts by weight of the polyolefin material) per minute. The grafting reaction is an exothermic reaction and is carried out in a conventional stirred reactor where heat transfer becomes the parameter which limits the ability to maintain good temperature control and productivity.
This heat transfer problem becomes magnified as you increase the size of the reactor since the surface to volume ratio gets smaller as you increase the size of the vessel in a gas phase process as there is no suspending or solvent medium to aid the heat transfer. Further, the faster you feed the monomer(s) to the reactor, the faster the heat is generated and the greater the heat transfer problems.
Another problem occurring in grafting reactions carried out in mechanically stirred reactors, derives from the effect of the agitator impacting the polymer particles, which contributes to fines formation and consequent fouling caused by the presence of dead zones with poor mixing action.